The long-term goal of this project is to develop strategies to improve the treatment of breast cancer. The aromatase inhibitors we pioneered in the early phases of this grant are now proving to be of value in the clinic. Although AIs appear to be more effective than tamoxifen, some patients may acquire resistance or have de novo resistance to AIs. The unique preclinical model we have developed to study the effect of aromatase inhibitors (AIs) has provided accurate predictions of clinical outcome. We propose to use this model to study mechanisms of tumor resistance. We have also developed unique tumors and cell lines that are resistant to letrozole and other AIs for use in these investigations. In the proposed studies, we will focus our efforts on several significant observations made during the current period. We plan to determine the mechanisms of resistance to aromatase inhibitors and how these might be reversed so that response to well tolerated AI treatment can be restored. The Specific Aims of the proposal are: 1) to investigate the functional the role of HER2/MAPK in estrogen receptor regulation and development of resistance to AIs. We will explore whether other inhibitors of HER2, such as lapatinib, pertuzumab and HKI-272 also reverse resistance to letrozole; 2) to investigate whether down-regulation of the estrogen receptor (ER) by fulvestrant prevents crosstalk with tyrosine kinase receptors (TKR) in AI resistance. We will determine whether combining an AI and estrogen down regulator is more effective than complete estrogen suppression in controlling tumor growth. Based on mechanisms identified, the optimal efficacy of these agents in combination or sequential strategies will be determined; 3) to investigate mechanisms involved in reversing resistance of tumors to letrozole and other AIs following withdrawal of AI treatment; 4) to investigate whether histone deacetylase (HDAC) inhibitors will convert ER negative cells to hormone responsive cells that are sensitive to HDACI + AI treatment and reduce the cancer progenitor cells or side population in these and AI resistant cells and tumors. Our studies should provide information with which to plan new strategies to treat breast cancer patients.